An image sensor may be a semiconductor device that converts an optical image to an electric signal. An image sensor may be classified as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) image sensor (CIS). In a CCD, respective MOS capacitors may be arranged close to one another and charge carriers may be stored in the capacitors and transferred. CMOS technology may use a control circuit and a signal processing circuit as peripheral circuits. A CMOS image sensor may include a number of MOS transistors corresponding to a number of pixels, and may detect outputs in sequence using the MOS transistors in a switching manner. A CCD may require a complicated operation system and manufacturing process. A CCD may also require high power consumption. Furthermore, since it may not be possible to form a signal processing circuit in a CCD chip, it may be difficult to implement a one-chip CCD. To overcome various problems of a CCD, a CMOS image sensor, which may apply a sub-micron CMOS manufacturing technology, may be used.
A CMOS image sensor may include a photodiode and a MOS transistor in a unit pixel. A CMOS image sensor may sequentially detect signals in a switching manner, and may thus determine an image. A CMOS image sensor may consume less power than a CCD, and may also be manufactured through a more simplified process. For example, it may require only about 20 masks to manufacture a CMOS image sensor, whereas a CCD manufacturing process may require 30 to 40 masks. Furthermore, a plurality of signal processing circuits may be embodied in one chip. Accordingly, a CMOS image sensor may be increasingly used in various applications, such as digital still cameras (DSC), PC cameras, mobile cameras, and so forth. A CMOS image sensor may be classified into 3T type, 4T type, 5T type, and so on, according to a number of transistors. A 3T type may include one photodiode and three transistors. A 4T type may include one photodiode and four transistors. A layout of a unit pixel of a 4T-type CMOS image sensor will be described below.
FIG. 1 is a drawing illustrating a layout of a related art 4T-type CMOS image sensor. FIG. 2 is a drawing illustrating a layout of a unit pixel of a related art 4T-type CMOS image sensor. Referring to FIG. 1 and FIG. 2, a unit pixel may include photodiode (P) 10, which may be an optical converter, and four transistors. The four transistors may include transfer transistor (Tx) 20, reset transistor (Rx) 30, drive or access transistor (Dx) 40, and select transistor (Sx) 50. A drain terminal of select transistor 50, which may be an output terminal of each unit pixel, may be electrically connected with a load transistor. Floating diffusion (FD) region 60 may also be provided. An operation processes of a related art CMOS image sensor may be as follows. First, reset transistor B 30 may be powered on, and an output floating diffusion node potential may become a power voltage VDD. A reference value may be detected. Upon incidence of light from outside of the image sensor to photodiode P 10, which may be a light receiving unit, electron-hole pairs (EHPs) may be generated in proportion to an intensity of the incident light. Potential of a source node of transfer transistor A 20 may be changed in proportion to an amount of generated signal charges.
When transfer transistor A 20 is powered on, accumulated signal charges may be transferred to floating diffusion region 60, and a potential of an output floating diffusion node may be changed in proportion to an amount of transferred signal charges. Simultaneously, a gate bias of select transistor D 50 may be changed. As a result, a change of source potential of driving transistor C 40 may occur. In this state, when driving transistor D may be powered on, data may be read out to a column. Reset transistor B 30 may be powered on, and an output floating diffusion node potential may become VDD. These processes may be repeated. At an upper part of respective photodiodes, a color filter array may be formed to separately receive red, green, and blue signals. In addition, a micro lens may be formed at an uppermost part of the light receiving unit, which may allow more light to be received. The signals of respective channels may be connected to an image processing circuit, which may be provided outside of the light receiving unit, through a plurality of metal lines, and may be combined into one image by signal processing.
FIG. 3A through FIG. 3C are drawings illustrating a shading effect that may occur in a related art CMOS image sensor, which may include photodiode 70, interlayer dielectric 72, color filter array 74, and micro lens 76. Referring to FIG. 3A through FIG. 3C, an incidence angle of light received through micro lens 76 may be varied according to a position of a pixel. An incidence angle of light being received at a center of a pixel area, as shown in FIG. 3A, may be different from an incidence angle of light being received at a periphery of the pixel area, as shown in FIG. 3B and FIG. 3C. An incidence angle may increase from a center toward a periphery. A shading effect may be caused due to such difference in incidence angles between the center and the periphery. Therefore, the periphery of an image may be shown darker than the center.